1. Field of the Invention
This invention relates to rubber compositions containing chlorinated polyethylene type rubbers and providing increased tensile modulus and improved metal adhesion. Such compositions are suitable for use in rubber-metal composite structures.
2. Description of the Prior Art
There is a growing tendency to use a variety of rubber products such as tires, belts, shapes, rolls, hoses and the like under adversely oily conditions at high temperatures and pressures and for long periods of time. In that instance, the fastly deteriorating rubbers require laborious maintenance and frequent replacement and sometimes pose serious problems or hazards.
Certain rubbers are known to be highly resistant to oil and heat for example at from 120.degree. to 150.degree. C. They include acrylonitrile-butadiene rubber (NBR), acrylic rubber (ACM), ethylene-acrylate rubber (AEM), ethylene-acrylate-vinyl acetate rubber (ER), chlorosulfonated polyethylene rubber (CSM), chlorinated polyethylene rubber (CM) and reformed rubbers such as NBR and similar rubbers having their conjugated dienes hydrogenated. Chlorinated polyethylene rubber (CM) amongst these rubbers provides a good balance between and among oxidation resistance, weather resistance, oil resistance and chemical resistance.
Chlorinated polyethylene rubber is by nature unvulcanizable with sulfur due to the absence of double bonds in the molecule. Vulcanization is effected for that reason with use of organic peroxides, thioureas, diamines and trithiocyanuric acids.
Thioureas and diamines, however, will make the resulting vulcanizates susceptible to insufficient modulus. Organic peroxides and trithiocyanuric acids, though giving improved modulus, will lead to reduced bonding to metals such as brass and the like. This causes a problem with the use of chlorinated polyethylene rubber.
In an attempt to fully utilize the physical characteristics associated with chlorinated polyethylene rubber, it has been proposed to blend a vulcanizing system composed of magnesia, epoxy resin, trially isocyanurate, dially phthalate, trithiocyanuric acid and organic peroxide as disclosed for instance in Japanese Patent Publication No. 61-26820. This prior system of vulcanization, however, requires so many components that it entails limited choices of formulations, and has thus failed to further improve the ultimate rubber composition in regard to its physical properties.